The invention concerns a projectile with a unit decreasing the base drag of the projectile and in the case of a gun launched projectile increases the range and decreases dispersion.
In order to increase the range of artillery projectiles, it has been suggested to arrange a rocket propulsion motor at the rear part of the projectile, thus creating what are generally known as RAP projectiles. However, the RAP system has a number of known disadvantages, since even minor changes in ignition time and rocket power cause considerable changes in range. It is difficult to control the exact point of ignition of the rocket charge which concomitantly causes difficulty in controlling the trajectory of the projectile. RAP-projectiles, even when seemingly otherwise similar and fired from the same point, land in a relatively widely dispersed area.
In order to overcome the disadvantages encountered with rocket propulsion assisted artillery projectiles, an alternative method has been suggested, namely, to reduce the base drag acting on the projectile during the trajectory. An artillery projectile in flight forms a low pressure area immediately behind the projectile which creates a force (commonly called "base drag") which lessens the velocity of the projectile. This method is a completely different approach to the problem, since no rocket assisting effect is achieved.
Base drag contributes generally to a relatively large part of the total drag and depends upon the fact that the base pressure due to the resulting wake flow in the base region is lower than the ambient air pressure. By ejecting a mass flow from the base region of the body into the near wake the flow pattern can be affected in such a way that the base pressure is increased and thereby the base drag is reduced. If the ejection of mass is combined with liberation of heat, e.g. by combustion, the base pressure can be further increased.
The above-mentioned effect, the base flow/burning effect, differs from rocket propulsion foremost therein that the reaction force caused by the base flow is very small as compared to the decrease in drag due to increased base pressure. In order to efficiently utilize the base flow effect the base mass flow should occur during a considerable part of the flight time.
Small caliber (e.g., about 30 mm. or less) projectiles have been equipped with tracer or fumer compositions which provide some relief to the base drag problems with these types of projectiles. See, for example, U.S. Pat. No. 3,086,009. These systems, however, burn relatively rapidly (usually for only a few seconds) and cannot be controlled to provide prolonged relief for longer times. Tracer or fumer compositions are generally solid, relatively inflexible compositions which also contain relatively high amounts of metals (e.g., about 35%) and thus produce a gas containing high amounts of solid combustion particles which do not provide relief to the base drag problem. Also, these systems are most often applicable to small caliber (e.g., about 30 mm or less), short range (e.g., about 4000 m., often about 300-400 m.) projectiles, although proposals for use with larger caliber projectiles have been made.
The problem with respect to base drag of a rocket motor and means for relieving base drag are disclosed in U.S. Pat. No. 3,885,385.
Generating the necessary base mass flow involves, e.g., in the case of a moving projectile, certain practical problems. The ejected mass has to be supplied from some system carried by the projectile.
Until the present invention, however, no satisfactory solution has been found for counteracting the effects of base drag on artillery projectiles.
The object of the present invention is to provide artillery projectiles arranged with a base drag reducing system, having a number of previously unknown features optimizing the achievable flight range and decreasing dispersion. This object has been achieved by a combination of certain design features relating to the projectile, and in particular the rear portion of the projectile, with an eminently suitable type of fuel and igniter charge.
In the present invention, the projectile for gun launching through a muzzle and having decreased base drag characteristics comprises a projectile body having a base including restricted gas outlet nozzle means and wall means interiorly of the projectile body which define a combustion chamber communicating with the gas outlet nozzle means. The projectile of the present invention also comprises base drag reduction means (including the combustion chamber, nozzle and propellant grain means of a fuel rich compostion located in the combustion chamber and presenting a burning area operable to be ignited upon exit from the muzzle of the gun) for ejecting a controlled low mass flow of gas into and liberating heat in the near wake zone of the body to increase the base pressure in the near wake zone and decrease the base drag of the gun projectile without rocket propulsion by exceeding ambient pressure in the wake zone. The exit area of the restricted outlet nozzle, the burning area and mass of the propellant and the static burning rate of the propellant are so related that the low mass flow of gas is operable to continue from the time of muzzle exit for a considerable portion of projectile flight and also that in flight the pressure in the combustion chamber only slightly exceed the pressure at the base in an amount of 0.01 to 0.5 bar. The combustion chamber is generally provided with an igniter system operable to maintain the ignition of the propellant upon exit from the muzzle.
In addition, total area of the base (A.sub.b), the area of the restricted gas outlet nozzle (A.sub.h) and the dimensionless mass flow rate of the ejected gas (I) has the ratio ##EQU1## of at least 10 at the muzzle.
The characteristics of the invention, devices, aspects and advantages will become apparent from the following description.